1. Field of the Invention
The present invention relates to a particle beam rotational irradiation apparatus (a rotating gantry), the objective of which is to irradiate a charged particle, accelerated by an accelerator, from an arbitrary angle direction.
2. Description of the Related Art
Charged particles are circulated and accelerated in a circular accelerator such as a synchrotron and then the charged particles (mainly protons or carbon ions), which have been accelerated to gain high energy, are extracted from the circulation orbit; the charged particles (referred to also as charged particle beam or particle beam) are utilized in a physics experiment or in a particle beam therapy such as a cancer treatment, in which the charged particles are transported through a beam transport line and irradiated onto a desired subject. In general, in a cancer treatment utilizing accelerated charged particles, i.e., in a so-called particle beam therapy, the irradiation directions are changed for the purpose of averting the charged particles from major organs during the treatment or preventing normal tissues from being damaged. As one of the means of changing the irradiation directions of the charged particle beams, a particle beam rotational irradiation apparatus (rotating gantry) is commonly utilized in which an irradiation nozzle is mounted in a structural member that rotates around a patient and the charged particles can be irradiated from a desired angle.
For example, in a rotating gantry (referred to simply as a gantry, as may be necessary) disclosed in Patent Document 1, a charged particle beam is once bended by two bending electromagnets in such a way that the beam transport line becomes perpendicular to the rotation axis of the rotating gantry; then, the charged particle beam is introduced by bending the charged particle beam again by use of two bending electromagnets in such a way that on a plane perpendicular to the center axis of the gantry, the charged particle beam is directed to the isocenter (the intersection point of the gantry rotation axis with the beam axis, which is the reference of the irradiation target). This kind of beam transport line makes the length of the rotating gantry shortest with respect to the rotation-axis direction; therefore, as a result, it is made possible to install a rotating gantry in a small area. In some cases, a rotating gantry provided with this kind of beam transport line is referred to as a corkscrew-type gantry. In addition, the bending electromagnets are two-pole electromagnets provided with two magnetic poles.
It is another characteristic of a corkscrew-type gantry that at least four bending electromagnets are required. The reason for that will be explained below. In general, the momentum of a charged particle beam extracted from a circular accelerator does not have a certain single value but has spread around the center value. The value obtained through dividing a deviation amount from the center value by the center value is referred to as a momentum spread. When a charged particle beam having a momentum spread passes through the bending electromagnet, the bending angle thereof changes depending on the momentum (it might be considered as energy or a velocity) of the charged particle beam; therefore, when the spread is left as it is, the width (referred to as a beam size, hereinafter) of particle distribution at the isocenter may become large.
Letting p0 and Δp/p0 denote the momentum (center momentum) of a particle having the center momentum at a given position and the momentum spread, respectively, the deviation (the spread of a beam width due to the momentum spread) Δx from the center orbit, which is the orbit of the particle having the center momentum p0, is given as the equation (1) by use of a dispersion function η that characterized the effect of the momentum spread at the given position. The dispersion function η is a function of the position of a beam transport line.Δx=η×Δp/p0  (1)
In general, as an element for causing the dispersion function η, a bending electromagnet is utilized; when the dispersion function η once becomes a value other than “0”, it is required to cancel η and η′ by use of at least another bending electromagnet and quadrupole electromagnets. Here, η′ denotes the differentiation in the beam traveling direction (s direction, s axis). Because an actual irradiation site is not a point like an isocenter and has a width in the depth direction, it is required to nullify the gradient η′ of the dispersion function η. A dynamic change of a momentum spread makes the charged particle beam look like moving. In general, it is required, in a rotating gantry, that the dispersion function η is diminished at the isocenter to the extent that its contribution to the spread of a beam width can be allowed, in order to prevent the beam width from changing or moving at the isocenter.
In the case of the corkscrew-type gantry disclosed in Patent Document 1, two bending electromagnets at the upstream side are situated on the same plane; therefore, the dispersion function η caused by the first bending electromagnet is nullified by the other bending electromagnet that has the bending plane thereof on the same plane. In this situation, a plurality of quadrupole electromagnets provided between the bending electromagnets are utilized for changing the s-direction gradient (η′) of the dispersion function η in addition to focusing or defocusing a charged particle beam. The bending planes of two bending electromagnets at the downstream side differ by 90 degrees from those of the two bending electromagnets at the upstream side; as is the case with the upstream side, η and η′ are nullified by the two electromagnets and a plurality of quadrupole electromagnets provided between the bending electromagnets. In general, in the designing of the beam transport line between a circular accelerator and a rotating gantry or in the designing of the beam transport line of a rotation gantry, there exists a type in which all bending electromagnets are arranged in a single bending direction and nullify η and η′, which are caused in only a single direction, in collaboration with quadrupole electromagnets, or a type, as disclosed in Patent Document 1, in which a plurality of bending electromagnets are arranged in such a way that although respective η and η′ are caused in both the x direction and the y direction, 90-degree-different bending planes makes the x-direction dispersion function η and the y-direction dispersion function η not couple with each other, i.e., the x-direction dispersion function η and the y-direction dispersion function η are independent in the respective directions thereof.
Next, a gantry in which η and η′ are caused in only a single direction will be explained. In the rotating gantry disclosed in Patent Document 2, three bending electromagnets lead a charged particle beam to the isocenter. Because three bending electromagnets are provided, the dispersion function is nullified in only one direction in the designing of the beam transport line of the rotating gantry, unless the coupling is not utilized; thus, the bending planes of a charged particle beam is made to be a single and the same by the three bending electromagnets. Accordingly, the beam transport line of the rotating gantry disclosed in Patent Document 2 becomes longer in the beam-rotation-axis direction than that of the corkscrew-type gantry; as a result, the area where the rotating gantry is installed becomes wider.
In recent years, it has been required to raise the throughput of particle beam therapy, as particle beam therapy has become widespread. In particle beam therapy, before a charged particle beam is irradiated onto a patient, a person who offers assistance in the therapy, for example, a radiologist approaches a patient platform or an irradiation nozzle and fixes the patient body or adjusts irradiation-system apparatuses that are to be mounted on an irradiation nozzle. In this situation, in order to shorten the time for the adjustment, i.e., in order to raise the throughput of the therapy, the easiness degree of the foregoing work at the vicinity of the irradiation nozzle is important. An example of rotating gantry that facilitates the work at the vicinity of the irradiation nozzle is disclosed, for example, in Patent Document 3. By constructing a rotating gantry (referred to as an open type, hereinafter) in such a way that as illustrated in FIG. 6 of Patent Document 3, the front end of the irradiation apparatus (irradiation nozzle) protrudes toward the irradiation room, the work to be performed in the vicinity of the irradiation nozzle can be facilitated. Moving body tracking, in which high-accuracy irradiation is performed while the motion of an organ undergoing irradiation or the motion of a body caused by respiration is monitored in real time, has also been attracting attention; therefore, it is desired to secure a sufficient space for arranging monitoring apparatuses in the vicinity of the irradiation nozzle.